1. Field of the Invention
The, present invention relates generally to rheology and in particular to improvements in optical rheometric testing.
2. Background of the Invention
In the rheology field the control of temperature of a sample under test is important because of the temperature sensitivity of properties such as viscoelastic behavior. In one known configuration, a sample is located between an upper and lower plate as illustrated in FIG. 1a. System 10 of FIG. 1a includes an upper rotatable plate 12 and a lower fixed plate 14. Upper plate 12 is configured to introduce a shear strain/stress into a fluid sample 13 in contact with plate 12 during plate rotation or oscillation. By simultaneous heating of the fluid sample the fluid rheological behavior can be studied as a function of temperature.
Many methods of fluid sample heating are known, such as forced convection, radiant heating, direct resistive heating of a lower plate, direct Peltier heating of a lower plate, and so forth. Each heating method when used in conjunction with rheometry has drawbacks due to limitations imposed by the specific apparatus used for heating, among other factors. For example, forced convection can introduce mechanical disturbances into the fluid sample under study. Radiant heating can introduce temperature gradients that make fluid temperature hard to control. Direct resistive heating of a lower plate and Peltier heating typically involve opaque metallic elements that preclude use of fluid sample optical measurements during rheometric measurements. For example, in known rheometers, a rotating plate, such as upper plate 12, can be fabricated using an optically transparent material to facilitate optical measurements of a sample fluid using an optical probe that passes through the sample and plate. However, if lower plate 14 is a heating plate that heats a sample placed above it using resistive or Peltier heating, plate 14 will block light so that an optical probe cannot easily access a heated fluid sample near plate 14. Accordingly, known rheometer systems in which sample heating is employed are limited in terms of measurement flexibility and control.